Background: Biomarkers play an essential role to inform treatment decisions in mBC. In hormone receptor positive/HER2 negative (HR+/HER2-) mBC, PIK3CA/AKT1/PTEN alterations are now actionable as pt selection biomarkers with the FDA’s Nov 2023 approval of the pan-AKT inhibitor capivasertib. Here we investigate the testing patterns and prevalence of PIK3CA/AKT1/PTEN alterations in mBC. Methods: This retrospective cohort study used two partially overlapping nationwide de-identified databases: the Flatiron Health electronic health record-derived database (FH DB) to assess next-generation sequencing (NGS) testing rates, and the Flatiron Health-Foundation Medicine clinico-genomic database (FH-FMI CGDB) to assess prevalence among tested. Pts were included if they were ≥18 years old and diagnosed with HR+/HER2- mBC from 1/1/17—6/30/22. Prevalence was assessed using any sample type or timing of testing post-mBC diagnosis and defined as pathogenic/likely pathogenic alterations as captured in FMI NGS testing. All analyses were summarized using descriptive statistics. Results: 8,049 (FH DB) and 2,912 (FH-FMI CGDB) pts were eligible. Overall, 37% of pts with mBC received NGS testing (3,002/8,049). Among pts in FH-FMI CGDB, 98% had PIK3CA tested (2,857/2,912) and 97% also had AKT1/PTEN tested (2,838/2,912) in the metastatic setting. The median time from mBC diagnosis to initial test was 148 days for PIK3CA and 188 days for AKT1/PTEN, and 92-93% of testing occurred after the start of first-line (1L) therapy (Table). 82% of PIK3CA testing was on tissue and 37% on liquid biopsy, with 19% of pts receiving both tissue and liquid biopsy. 77% of AKT1/PTEN testing was on tissue and 28% on liquid biopsy, with 5% of pts receiving both tissue and liquid biopsy. At least one PIK3CA/AKT1/PTEN alteration was detected in 55% of the pts (1,557/2857) tested for any of the markers, regardless of sample type. Conclusions:<40% of pts with mBC received NGS testing from 2017-2021, suggesting opportunities for improving pts’ access to testing. Coupled with the high rate of PIK3CA/AKT1/PTEN alterations, NGS testing should be incorporated in routine clinical practice to identify optimal treatments.

*227 pts did not have 1L.

⁺including co-alterations among PIK3CA/AKT1/PTEN.